Cutting head having tip portion with radially extending front cutting edges provided with both negative and positive rake angles, and rotary cutting tool

ABSTRACT

A cutting head rotatable about a first axis, comprising an intermediate portion and a tip portion. The intermediate portion has a plurality of leading edges defining a cutting diameter, and the tip portion has an axially forwardmost tip point and a plurality of front surfaces with outer and inner cutting edges. An outer rake surface adjacent to each outer cutting edge has a positive outer rake angle, and an inner rake surface adjacent to each inner cutting edge has a negative inner rake angle. Each outer rake surface is disposed on a head flute intersecting one of the leading edges, and each inner rake surface is disposed on a gash intersecting one of the head flutes. Each gash extends to a gash path end point located a first distance axially rearward of the tip point, and the first distance is greater than thirty percent of the cutting diameter.

RELATED APPLICATIONS

The present application claims priority to U.S. Provisional applicationNo. 62/741,000, filed Oct. 4, 2018. The contents of the aforementionedapplication are incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to a cutting head having a tip portionwith radially extending cutting edges and a rotary cutting tool havingsuch cutting head, for use in metal cutting processes in general, andfor drilling operations in particular.

BACKGROUND OF THE INVENTION

Within the field of cutting tools used in drilling operations, there aremany examples of cutting heads having cutting edges configured toaccount for the increased wear at radially outer portions due to therelatively higher cutting speeds, and/or reduced stability at radiallyinner portions due to the relatively higher cutting forces.

U.S. Pat. No. 8,801,344 discloses a drill bit having at least one maincutting edge and at least one center cutting edge, wherein the drill bitcomprises a longitudinal axis, and wherein the at least one main cuttingedge and the at least one center cutting edge are each assigned a rakeface. The drill bit is characterized in that the rake face assigned tothe at least one center cutting edge has at least two part faces whichas seen as perpendicular to the longitudinal axis of the drill bit forman obtuse angle with one another, so that the at least one centercutting edge comprises at least two part cutting edges.

WO 2018/075921 A1 discloses a drill including a plurality of lands thatextend to a cutting edge, where adjacent lands are separated by flutescomprising a base contour arranged in a generally helical configurationalong a centerline axis of a drill body. The drill also includes aplurality of contoured drill points each having a linear portion thatextends towards an outer diameter of the drill body, and an arcuateportion that extends from the linear portion and towards a chisel of thedrill body. The drill further includes a plurality of gash contourspositioned within the plurality of flutes. The gash contours extend fromthe chisel of the drill body, and the gash contours are oblique to thebase contours of the flutes.

WO 2018/079489 A1 discloses a cutting tool with a rod-shaped body, acutting blade located at a first end of the body, and a groove thatextends in a spiral from the cutting blade toward a second end side ofthe body. The cutting blade comprises a first blade intersecting with anaxis of rotation when seen in front view, and a second blade extendingfrom the first blade toward an outer peripheral surface of the body. Thegroove comprises a first thinning section located so as to connect tothe first blade, and a second thinning section located so as to connectto the second blade. A thinning angle of the first thinning section issmaller than a thinning angle of the second thinning section.

It is an object of the present invention to provide an improved cuttinghead having radially outer cutting edges with greater wear resistanceand radially inner cutting edges with increased stability androbustness.

It is also an object of the present invention to provide an improvedcutting head having gashes adjacent the radially inner cutting edgeswhich provide efficient chip evacuation.

It is a further object of the present invention to provide an improvedcutting head capable of operating at high feed rates.

It is yet a further object of the present invention to provide animproved rotary cutting tool in which the cutting head is removablymounted to a shank.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a cuttinghead rotatable about a first axis in a direction of rotation, andcomprising:

an intermediate portion having an integer number N, N≥2,circumferentially spaced apart peripheral surfaces, each peripheralsurface having a leading edge, and the plurality of leading edgesdefining a cutting diameter; and

a tip portion having an axially forwardmost tip point contained in thefirst axis and N front surfaces, each front surface having a radiallyextending front cutting edge which comprises an outer cutting edgeextending radially inwardly from one of the leading edges and an innercutting edge extending radially inwardly from said outer cutting edge,each inner cutting edge adjoining its associated outer cutting edge at acutting edge transition point,

wherein:

in a cross-section taken in a first vertical plane parallel to the firstaxis and intersecting any one of the outer cutting edges, an outer rakesurface adjacent to said outer cutting edge is inclined at a positiveouter rake angle; and

in a cross-section taken in a second vertical plane parallel to thefirst axis and intersecting any one of the inner cutting edges, an innerrake surface adjacent to said inner cutting edge is inclined at anegative inner rake angle,

wherein:

each outer rake surface is disposed on a head flute extending axiallyrearwardly from the tip portion and intersecting one of the leadingedges; and

each inner rake surface is disposed on a gash extending axiallyrearwardly from the tip portion and intersecting one of the head flutes,

and wherein:

each gash has a gash path defined by a plurality of gash apex pointsfrom a series of cross-sections taken in planes perpendicular to thefirst axis and intersecting the gash along its axial extent;

each gash path extends to a gash path end point located a first distanceaxially rearward of the tip point; and

the first distance is greater than thirty percent of the cuttingdiameter.

Also, in accordance with the present invention, there is provided arotary cutting tool comprising the cutting head described above and ashank having a longitudinal axis, and N shank flutes circumferentiallyalternating with N lands.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding, the invention will now be described, by wayof example only, with reference to the accompanying drawings in whichchain-dash lines represent cut-off boundaries for partial views of amember and in which:

FIG. 1 is a perspective view of a cutting head in accordance with thepresent invention;

FIG. 2 is a side view of the cutting head shown in FIG. 1;

FIG. 3 is a top view of the cutting head shown in FIG. 1;

FIG. 4 is a cross-sectional view of the cutting head shown in FIG. 3,taken along the line IV-IV;

FIG. 5 is a cross-sectional view of the cutting head shown in FIG. 3,taken along the line V-V;

FIG. 6 is a cross-sectional view of the cutting head shown in FIG. 2,taken along the line VI-VI;

FIG. 7 is a cross-sectional view of the cutting head shown in FIG. 2,taken along the line VII-VII;

FIG. 8 is a cross-sectional view of the cutting head shown in FIG. 3,taken along the line VIII-VIII;

FIG. 9 is a perspective view of a rotary cutting tool in accordance withthe present invention; and

FIG. 10 is an exploded view of the rotary cutting tool shown in FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

Attention is first drawn to FIGS. 1 to 3, showing a cutting head 20which may be manufactured by form pressing and sintering a cementedcarbide, such as tungsten carbide, and may be coated or uncoated.

According to the present invention, the cutting head 20 is rotatableabout a first axis A1 in a direction of rotation DR, comprising anintermediate portion 22 and a tip portion 24.

As shown in FIGS. 1 to 3, the intermediate portion 22 has a plurality ofN circumferentially spaced apart peripheral surfaces 26. Each peripheralsurface 26 has a leading edge 28, and the plurality of leading edges 28define a cutting diameter DC.

In some embodiments of the present invention, each leading edge 28 mayextend opposite the direction of rotation DR as it extends axiallyrearwardly from the tip portion 24.

Also, in some embodiments of the present invention, each leading edge 28may extend helically along the first axis A1.

As shown in FIGS. 1 to 3, the tip portion 24 has an axially forwardmosttip point NT contained in the first axis A1 and a plurality of N frontsurfaces 30, each front surface 30 having a radially extending frontcutting edge 31 which comprises an outer cutting edge 32 extendingradially inwardly from one of the leading edges 28 and an inner cuttingedge 34 extending radially inwardly from said outer cutting edge 32.

Each front surface 30 also includes a clearance surface 36 adjacent itsassociated outer and inner cutting edges 32, 34, and each inner cuttingedge 34 adjoins its associated outer cutting edge 32 at a cutting edgetransition point NR. As discussed below, the outer cutting edge 32 isassociated with a positive rake angle while the inner cutting edge 34 isassociated with a negative rake angle. Thus, the cutting edge transitionpoint NR corresponds to the point on the front cutting edge 31 where therake angle changes from a positive rake to a negative rake, whiletraveling along the front cutting edge 31 in a radially inward directiontowards the forwardmost tip point NT.

As shown in FIG. 3, the plurality of cutting edge transition points NRdefine an imaginary first circle C1 having a first diameter D1.

In some embodiments of the present invention, the first diameter D1 maybe greater than thirty percent of the cutting diameter DC, i.e.D1>0.30*DC.

As shown in FIGS. 1 to 3, the tip portion 24 may also include aplurality of N chisel edges 38, each chisel edge 38 being formed by twoadjacent clearance surfaces 36 and extending radially away from the tippoint NT to one of the inner cutting edges 34.

It should be appreciated throughout the description and claims, that Nis an integer that is at least two, i.e., N≥2.

In some embodiments of the present invention, the cutting head 20 mayexhibit N-fold rotational symmetry about the first axis A1.

Also, in some embodiments of the present invention, N may equal 3, andthe intermediate portion 22 may have three leading edges 28, and the tipportion 24 may have three outer cutting edges 32 and three inner cuttingedges 34.

Having three outer cutting edges 32 and three inner cutting edges 34enables the cutting head 20 to operate at high feed rates.

As shown in FIG. 4, in a cross-section taken in a first vertical planePV1 parallel to the first axis A1 and intersecting any one of the outercutting edges 32, an outer rake surface 40 adjacent to said outercutting edge 32 is inclined at a positive outer rake angle α1.

It should be appreciated that the expression “vertical plane”, as usedin the present application, refers to any plane that is parallel to thefirst axis A1, though not necessarily containing the first axis A1.

It should be appreciated throughout the description and claims, that theterm “rake angle” refers to the acute angle formed between a rakesurface and an imaginary reference line parallel to the first axis A1.

It should also be appreciated that the outer cutting edges 32 aresusceptible to greater wear than the inner cutting edges 34 due to theirrelatively higher cutting speeds, and that configuring the outer rakeangle α1 to be positive reduces wear on the outer cutting edges 32, thusprolonging the operative life thereof.

As shown in FIG. 3, the plurality of outer rake surfaces 40 face thedirection of rotation DR.

In some embodiments of the present invention, in a cross-section takenin any plane parallel to the first axis A1 and intersecting any one ofthe outer cutting edges 32, the outer rake surface 40 adjacent to saidouter cutting edge 32 may be inclined at a positive outer rake angle α1.

Also, in some embodiments of the present invention, in the cross-sectiontaken in the first vertical plane PV1, the positive outer rake angle α1may have a magnitude of greater than 5 degrees, while in someembodiments the positive outer rake angle α1 may have a magnitude ofgreater than 10 degrees.

As shown in FIG. 4, in the cross-section taken in the first verticalplane PV1, the clearance surface 36 is inclined at a positive outerclearance angle β1.

It should be appreciated throughout the description and claims, that theterm “clearance angle” refers to the acute angle formed between aclearance surface and an imaginary reference line perpendicular to thefirst axis A1.

As shown in FIG. 5, in a cross-section taken in a second vertical planePV2 parallel to the first axis A1 and intersecting any one of the innercutting edges 34, an inner rake surface 42 adjacent to said innercutting edge 34 is inclined at a negative inner rake angle α2.

It should be appreciated that the inner cutting edges 34 are susceptibleto greater impact forces than the outer cutting edges 32 due to theirrelatively lower cutting speeds, especially at high feed rates, and thatconfiguring the inner rake angle α2 to be negative increases thestability and robustness of the inner cutting edges 34, thus prolongingthe operative life thereof.

As shown in FIG. 3, the plurality of inner rake surfaces 42 face thedirection of rotation DR.

In some embodiments of the present invention, in a cross-section takenin any plane parallel to the first axis A1 and intersecting any one ofthe inner cutting edges 34, the inner rake surface 42 adjacent to saidinner cutting edge 34 may be inclined at a negative inner rake angle α2.

Also, in some embodiments of the present invention, in the cross-sectiontaken in the second vertical plane PV2, the negative inner rake angle α2may have a magnitude of greater than 5 degrees.

As shown in FIG. 5, in the cross-section taken in the second verticalplane PV2, the clearance surface 36 is inclined at a positive innerclearance angle β2.

In some embodiments of the present invention, the inner clearance angleβ2 may be greater than the outer clearance angle β1, i.e. β2>β1.

Also, in some embodiments of the present invention, the inner clearanceangle β2 may continuously increase when measured at a series of parallelcross-sections taken in parallel vertical planes located progressivelycloser to the first axis A1.

Configuring the inner clearance angle β2 to continuously increaseradially inwardly, reduces the high cutting and impact forces typicallyassociated with very low cutting speeds, occurring towards the cuttinghead's center.

As shown in FIGS. 1 to 3, each outer rake surface 40 is disposed on ahead flute 44 extending axially rearwardly from the tip portion 24 andintersecting one of the leading edges 28, and each inner rake surface 42is disposed on a gash 46 extending axially rearwardly from the tipportion 24 and intersecting one of the head flutes 44.

Also, as shown in FIGS. 1 to 3, each gash 46 has a gash path GP definedby a plurality of gash apex points from a series of parallelcross-sections taken in parallel planes, each of which is perpendicularto the first axis A1 and intersects the gash 46 along its axial extent.

It should be appreciated throughout the description and claims, that foreach cross-section taken in a plane perpendicular to the first axis A1and intersecting the gash 46, an associated gash apex point is locatedat the midpoint of a segment of the associated profile having a minimumradius, the minimum radius having a tolerance of +0.20/−0.00 mm.

According to the present invention, as shown in FIG. 2, each gash pathGP extends to a gash path end point NP located a first distance d1axially rearward of the tip point PT, and the first distance d1 isgreater than thirty percent of the cutting diameter DC, i.e. d1>0.30*DC.

Configuring each gash path GP to have an extensive axial length, by wayof the first distance d1 being greater than thirty percent of thecutting diameter DC, advantageously contributes to increased gash volumeand efficient chip evacuation.

In some embodiments of the present invention, the first distance d1 maybe greater than forty percent of the cutting diameter DC, i.e.d1>0.40*DC.

Also, in some embodiments of the present invention, each gash path GPmay extend in a direction opposite to the direction of rotation DR as itextends axially rearwardly from the tip portion 24.

Further, in some embodiments of the present invention, each gash pathend point NP may be located radially further from the first axis A1 thanany of the cutting edge transition points NR.

Configuring each gash path end point NP to be located radially outwardof the cutting edge transition points NR promotes improved chipdevelopment along the gash 46.

As shown in FIG. 6, in a cross-section taken in a first horizontal planePH1 perpendicular to the first axis A1 and intersecting the plurality ofinner cutting edges 34, each gash 46 may have a concave shaped firstprofile P1.

In some embodiments of the present invention, each first profile P1 maybe continuously curved.

Configuring each first profile P1 to be continuously curved, promotesimproved chip development in the associated gash region.

As shown in FIG. 6, the first profile P1 has a minimum first radius R1measured along a first segment S1 thereof, the first segment S1containing a first gash apex point NA1.

In some embodiments of the present invention, the minimum first radiusR1 may be greater than six percent of the cutting diameter DC, i.e.R1>0.06*DC.

Configuring each first profile P1 to have its minimum first radius R1greater than six percent of the cutting diameter DC promotes smooth chipflow along the gash 46, and a reduced risk of chip clogging.

Also, configuring each first profile P1 to have its minimum first radiusR1 greater than six percent of the cutting diameter DC increases thecore strength of the tip portion 24.

In some embodiments of the present invention, the minimum first radiusR1 may preferably be greater than eight percent of the cutting diameterDC, i.e. R1>0.08*DC.

Also, in some embodiments of the present invention, the minimum firstradius R1 may be less than fifteen percent of the cutting diameter DC,i.e. R1<0.15*DC.

As shown in FIG. 6, each first profile P1 may have a radially innermostfirst point NI1 contained in its first segment S1.

In some embodiments of the present invention, the first segment S1 maysubtend an angle of greater than 15 degrees about a first center pointE1 of the minimum first radius R1.

Configuring each first profile P1 to have its radially innermost firstpoint NI1 in the first segment S1, enables more efficientcircumferential spacing of the plurality of gashes 46, thus enablingcutting head configurations where N is greater than 2, i.e. N>2.

As shown in FIG. 7, in a cross-section taken in a second horizontalplane PH2 perpendicular to the first axis A1 and intersecting theplurality of leading edges 28, each gash 46 may have a concave shapedsecond profile P2.

In some embodiments of the present invention, each second profile P2 maybe continuously curved.

Configuring each second profile P2 to be continuously curved, promotesimproved chip development in the associated gash region.

As shown in FIG. 7, the second profile P2 has a minimum second radius R2measured along a second segment S2 thereof, the second segment S2containing a second gash apex point NA2.

In some embodiments of the present invention, the minimum second radiusR2 may be greater than six percent of the cutting diameter DC, i.e.R2>0.06*DC.

Configuring each second profile P2 to have its minimum second radius R2greater than six percent of the cutting diameter DC promotes smooth chipflow along the gash 46, and a reduced risk of chip clogging.

In some embodiments of the present invention, the minimum second radiusR2 may preferably be greater than eight percent of the cutting diameterDC, i.e. R2>0.08*DC.

Also, in some embodiments of the present invention, the minimum secondradius R2 may be less than fifteen percent of the cutting diameter DC,i.e. R2<0.15*DC.

It should be appreciated that the minimum second radius R2 may have arange of between eighty five and one hundred and fifteen percent of theminimum first radius R1, i.e. 0.85*R1<R2<1.15*R1.

As shown in FIG. 7, each second profile P2 may have a radially innermostsecond point NI2 contained in its second segment S2.

In some embodiments of the present invention, the second segment S2 maysubtend an angle of greater than 15 degrees about a second center pointE2 of the minimum second radius R2.

Configuring each second profile P2 to have its radially innermost secondpoint NI2 in the second segment S2, enables more efficientcircumferential spacing of the plurality of gashes 46, thus enablingcutting head configurations where N is greater than 2, i.e. N>2.

For embodiments of the present invention in which N is equal to 3, asshown in FIGS. 6 and 7, the first profile P1 may form a pursuit curvehaving a first start point NS1 located rotationally ahead of a first endpoint NE1, and the second profile P2 may form a pursuit curve having asecond start point NS2 located rotationally behind a second end pointNE2.

It should be appreciated that use of the term “pursuit curve” throughoutthe description and claims refers to the curve shape described inhttps://en.wikipedia.org/wiki/Pursuit_curve, retrieved Jul. 2, 2019, thecurve being traced by a pursuer in pursuit of a pursuee, with thepursuee moving in a straight line and always on the pursuer's tangent.

As seen in FIG. 1, the head flute 44 and its associated gash 46 meetalong a gash-flute boundary line GFB. As shown in FIG. 7, in across-section taken in a third horizontal plane PH3 perpendicular to thefirst axis A1 and intersecting the plurality of leading edges 28, eachgash 46 intersects its associated head flute 44 at a gash-fluteintersection point IG, the gash-flute boundary line GFB, constituting acollection of such gash-flute intersection points IG at various suchhorizontal planes.

In some embodiments of the present invention, each such gash-fluteintersection point IG, except at the associated cutting edge transitionpoint NR itself, may be located rotationally ahead of its associatedcutting edge transition point NR.

Also, in some embodiments of the present invention, the second and thirdhorizontal planes PH2, PH3 may be coplanar.

As shown in FIG. 8, in a cross-section taken in a third vertical planePV3 containing the first axis A1 and intersecting the clearance surface36, the clearance surface 36 may have a concave shaped clearance profilePC.

In some embodiments of the present invention, each concave shapedclearance profile PC may have a clearance radius RC having a range ofbetween fifty and one hundred and fifty percent of the cutting diameterDC, i.e. 0.50*DC<RC<1.50*DC.

Also, in some embodiments of the present invention, each concave shapedclearance profile PC may be continuously curved and extend step-free tothe first axis A1.

Attention is now drawn to FIGS. 9 and 10, showing a rotary cutting tool48 according to the present invention, comprising the cutting head 20and a shank 50 having a longitudinal axis L.

The shank 50 has N shank flutes 52 circumferentially alternating with Nlands 54, and each shank flute 52 may extend helically along thelongitudinal axis L.

As shown in FIGS. 9 and 10, the cutting head 20 may have an axiallyrearward facing bottom surface 56, the shank 50 may have a supportsurface 58 transverse to the longitudinal axis L, and the cutting head20 may be removably mounted to the shank 50 with the bottom surface 56in contact with the support surface 58.

Configuring the cutting head 20 to be removably mounted to the shank 50enables the cutting head 20 to be manufactured from a suitably hardmaterial, such as tungsten carbide, and the shank 50 to be manufacturedfrom a less hard and less expensive material, such as high-speed steel.The shank 50 may be reusable following disposal of a worn or damagedcutting head 20.

In some embodiments of the present invention, each head flute 44 mayintersect the bottom surface 56 and cooperate with one of the shankflutes 52.

Also, in some embodiments of the present invention, the bottom surface56 may be perpendicular to the first axis A1, the support surface 58 maybe perpendicular to the longitudinal axis L, and the first axis A1 maybe coaxial with the longitudinal axis L.

As shown in FIG. 2, the bottom surface 56 is located a second distanced2 axially rearward of the tip point PT, and the first distance d1 maybe greater than seventy percent of the second distance d2, i.e.d1>0.70*d2.

In some embodiments of the present invention, the cutting head 20 mayinclude a mounting protuberance 60 extending axially rearwardly from thebottom surface 56.

In other embodiments of the present invention (not shown), the cuttinghead 20 and the shank 50 may be integral parts of a unitary one-piececonstruction, and each head flute 44 may merge with one of the shankflutes 52.

As shown in FIGS. 9 to 10, the intermediate portion 22 of the cuttinghead 20 may include a plurality of N torque transmission surfaces 62facing opposite the direction of rotation DR, the shank 50 may include aplurality of N drive protuberances 64, with each drive protuberance 64having a drive surface 66 facing the direction of rotation DR, and eachtorque transmission surface 62 may be in contact with one of the drivesurfaces 66.

In some embodiments of the present invention, the first distance d1 maybe less than ninety percent of the second distance d2, i.e. d1<0.90*d2.

For embodiments of the present invention in which N is equal to 3,configuring the first distance d1 to be less than ninety percent of thesecond distance d2, provides sufficient space for the plurality of driveprotuberances 64 to engage the cutting head 20, without obstructingsmooth chip flow between the gashes 46 and the shank flutes 52.

In some embodiments of the present invention, each torque transmissionsurface 62 may intersect one of the peripheral surfaces 26.

For embodiments of the present invention in which N is equal to 3, asshown in FIGS. 1 to 3, each gash 46 may intersect one of the torquetransmission surfaces 62 at a radially outermost gash point NO.

In some embodiments of the present invention, as shown in FIG. 3, thethree radially outermost gash points NO may define an imaginary secondcircle C2 having a second diameter D2 greater than seventy percent ofthe cutting diameter DC, i.e. D2>0.70*DC.

Configuring the imaginary second circle C2 to have a second diameter D2greater than seventy percent of the cutting diameter DC, advantageouslycontributes to increased gash volume and efficient chip evacuation.

Although the present invention has been described to a certain degree ofparticularity, it should be understood that various alterations andmodifications could be made without departing from the spirit or scopeof the invention as hereinafter claimed.

What is claimed is:
 1. A cutting head (20) rotatable about a first axis(A1) in a direction of rotation (DR), and comprising: an intermediateportion (22) having an integer number N, N≥2 circumferentially spacedapart peripheral surfaces (26), each peripheral surface (26) having aleading edge (28), and the plurality of leading edges (28) defining acutting diameter (DC); and a tip portion (24) having an axiallyforwardmost tip point (NT) contained in the first axis (A1) and N frontsurfaces (30), each front surface (30) having a radially extending frontcutting edge (31) which comprises an outer cutting edge (32) extendingradially inwardly from one of the leading edges (28) and an innercutting edge (34) extending radially inwardly from said outer cuttingedge (32), each inner cutting edge (34) adjoining its associated outercutting edge (32) at a cutting edge transition point (NR), wherein: in across-section taken in a first vertical plane (PV1) parallel to thefirst axis (A1) and intersecting any one of the outer cutting edges(32), an outer rake surface (40) adjacent to said outer cutting edge(32) is inclined at a positive outer rake angle (α1); and in across-section taken in a second vertical plane (PV2) parallel to thefirst axis (A1) and intersecting any one of the inner cutting edges(34), an inner rake surface (42) adjacent to said inner cutting edge(34) is inclined at a negative inner rake angle (α2), wherein: eachouter rake surface (40) is disposed on a head flute (44) extendingaxially rearwardly from the tip portion (24) and intersecting one of theleading edges (28); and each inner rake surface (42) is disposed on agash (46) extending axially rearwardly from the tip portion (24) andintersecting one of the head flutes (44), and wherein: each gash (46)has a gash path (GP) defined by a plurality of gash apex points from aseries of cross-sections taken in planes perpendicular to the first axis(A1) and intersecting the gash (46) along its axial extent; each gashpath (GP) extends to a gash path end point (NP) located a first distance(d1) axially rearward of the tip point (PT); and the first distance (d1)is greater than thirty percent of the cutting diameter (DC).
 2. Thecutting head (20) according to claim 1, wherein: in a cross-sectiontaken in a first horizontal plane (PH1) perpendicular to the first axis(A1) and intersecting the plurality of inner cutting edges (34), eachgash (46) has a concave shaped first profile (P1); the first profile(P1) has a minimum first radius (R1) measured along a first segment (S1)thereof, the first segment (S1) containing a first gash apex point(NA1); and the minimum first radius (R1) is greater than six percent ofthe cutting diameter (DC).
 3. The cutting head (20) according to claim2, wherein: each first profile (P1) has a radially innermost first point(NI1) contained in its first segment (S1).
 4. The cutting head (20)according to claim 2, wherein: each first profile (P1) is continuouslycurved.
 5. The cutting head (20) according to claim 2, wherein: in across-section taken in a second horizontal plane (PH2) perpendicular tothe first axis (A1) and intersecting the plurality of leading edges(28), each gash (46) has a concave shaped second profile (P2), thesecond profile (P2) has a minimum second radius (R2) measured along asecond segment (S2) thereof, the second segment (S2) containing a secondgash apex point (NA2); and the minimum second radius (R2) is greaterthan six percent of the cutting diameter (DC).
 6. The cutting head (20)according to claim 5, wherein: each second profile (P2) has a radiallyinnermost second point (NI2) contained in its second segment (S2). 7.The cutting head (20) according to claim 5, wherein: each second profile(P2) is continuously curved.
 8. The cutting head (20) according to claim1, wherein: in a cross-section taken in any vertical plane parallel tothe first axis (A1) and intersecting any one of the outer cutting edges(32), the outer rake surface (40) adjacent to said outer cutting edge(32) is inclined at a positive outer rake angle (al).
 9. The cuttinghead (20) according to claim 1, wherein: in a cross-section taken in anyvertical plane parallel to the first axis (A1) and intersecting any oneof the inner cutting edges (34), the inner rake surface (42) adjacent tosaid inner cutting edge (34) is inclined at a negative inner rake angle(α2).
 10. The cutting head (20) according to claim 1, wherein: in thecross-section taken in the second vertical plane (PV2), the negativeinner rake angle (α2) has a magnitude of greater than 5 degrees.
 11. Thecutting head (20) according to claim 1, wherein: each gash path (GP)extends in a direction opposite the direction of rotation (DR), as itextends axially rearwardly from the tip portion (24).
 12. The cuttinghead (20) according to claim 1, wherein: each gash path end point (NP)is located radially further from the first axis (A1) than any one of thecutting edge transition points (NR).
 13. The cutting head (20) accordingto claim 1, wherein: each front surface (30) includes a clearancesurface (36) adjacent its associated outer and inner cutting edges (32,34); and in a cross-section taken in a third vertical plane (PV3)containing the first axis (A1) and intersecting the clearance surface(36), the clearance surface (36) has a concave shaped clearance profile(PC).
 14. The cutting head (20) according to claim 13, wherein: eachconcave shaped clearance profile (PC) is continuously curved and extendsstep-free to the first axis (A1).
 15. The cutting head (20) according toclaim 13, wherein: the tip portion (24) includes N chisel edges (38),and each chisel edge (38) is formed by two adjacent clearance surfaces(36) and extends radially away from the tip point (NT) to one of theinner cutting edges (34).
 16. The cutting head (20) according to claim1, wherein: the plurality of cutting edge transition points (NR) definean imaginary first circle (C1) having a first diameter (D1), and thefirst diameter (D1) is greater than thirty percent of the cuttingdiameter (DC).
 17. The cutting head (20) according to claim 1, wherein:the cutting head (20) exhibits N-fold rotational symmetry about thefirst axis (A1).
 18. The cutting head (20) according to claim 1, whereinN=3.
 19. A rotary cutting tool (48) comprising: the cutting head (20)according to claim 1; and a shank (50) having a longitudinal axis (L),and N shank flutes (52) circumferentially alternating with N lands (54).20. The rotary cutting tool (48) according to claim 19, wherein: thecutting head (20) has an axially rearward facing bottom surface (56),the shank (50) has a support surface (58) transverse to the longitudinalaxis (L), and the cutting head (20) is removably mounted to the shank(50) with the bottom surface (56) in contact with the support surface(58).